The antagonists could be dark guild members. They might hear about the special baby that Lucy and Laxus are involved with. Maybe the baby has some sort of powerful magic within it, and the dark guild wants to steal the baby for their own evil purposes. So they constantly try to attack Lucy and Laxus to get their hands on the baby.
Another option for antagonists could be some of the more traditional - minded members of the guild who don't approve of Lucy and Laxus being together and having a baby. They might try to create problems for the couple, like spreading rumors or trying to separate them. This would add a lot of internal conflict to the fanfiction as Lucy and Laxus have to deal with the disapproval while also taking care of the baby.
Rittenhouse could be antagonists. They might see the baby as a tool or a threat to their plans for controlling time. They could try to kidnap the baby to use its potential powers for their own evil purposes.
One possible plot could be that Lucy and Laxus accidentally end up having to take care of a baby together. Maybe they find a lost baby during one of their guild missions. At first, they're both really clumsy and unsure of how to handle it. But as time goes on, they start to develop a bond not only with the baby but also with each other in a new, more family - like way.
Maybe a rival guild that is jealous of Fairy Tail's new addition. They might think that having a baby with such strong magical potential in Fairy Tail gives them an unfair advantage. So they try to kidnap the baby to either keep it for themselves or use it to bargain with Fairy Tail.
One possible plot could be that they accidentally end up having to take care of a baby during a mission. Maybe they find an orphaned baby in a dangerous situation and decide to protect it. This could lead to some really cute and funny moments as they struggle to take care of the baby's needs while also dealing with the threats around them.
His guildmates would surely be main characters. For example, Natsu could be there. Natsu might be really confused at first but then would be super protective of baby Laxus. Another could be Lucy. She might be the one who is more practical in taking care of the baby, like making sure he has proper clothes and a comfortable place to sleep.
The Vicious 6 could be the antagonists. They might see Gru's preoccupation with Lucy's pregnancy as a weakness and try to take advantage of it. They could plan to kidnap Lucy to get to Gru and force him to give up his new - found family life and return to his old villainous ways.
For Laxus, he might become more gentle and patient. Usually seen as a tough and somewhat brash character, having a baby could soften him. He would learn to control his temper better around the baby. For Lucy, she would likely become more confident in her maternal abilities. She has always been kind - hearted, but now she has to be strong for the baby. She might also develop new magical skills or ways to protect the baby.
One common theme could be family building. Since there's a 'baby' involved, it often explores how Laxus and Lucy adapt to becoming parents and building a family together. Another theme might be the growth of their relationship. Having a baby can put a strain or strengthen their bond, and this is often explored in such fanfiction.
Well, perhaps Laxus has some misguided notion that Lucy has information vital to the guild's safety. So he kidnaps her thinking he can extract it from her quickly. But in reality, Lucy has no such information and she has to convince Laxus of that.
It could be powerful fairies who have a vendetta against Lucy for being a slayer. Maybe they are the leaders of a fairy clan who see her as a threat to their existence.